I'm trying out this circuit with the difference that mine is 3.3V instead of 5V and I do not have the resistor (pot in schematic), just straight to mcu.
The odd behavior is that when the probes are not connected in any way, I'm getting an ADC reading of about 3900 (12 bit ADC).
When I put a damp paper towel between the probes, I get 4095.
I would expect around 0 when nothing is connecting the probes. What could be wrong?